We need a way to send an IPI (remote or local) in order toasynchronously restart the tick for CPUs in nohz adaptive mode.

This must be asynchronous such that we can trigger it with irqsdisabled. This must be usable as a self-IPI as well for examplein cases where we want to avoid random dealock scenario whilerestarting the tick inline otherwise.

This only settles the x86 backend. The core tick restart functionwill be defined in a later patch.

[CHECKME: Perhaps we instead need to use irq work for self IPIs.But we also need a way to send async remote IPIs.]